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I can identify hinge flaps. They're pretty clear. They just rotate on a single axis within the body of the flap. My question deals with the vague grey area between slotted and Fowler flaps. At the beginning of the scale is the design where there are 2 or 3 tabs protruding down from the trailing edge to provide a rotation point that creates a turning radius with the axis outside the flap body. At the end are jumbo-jet-style Fowlers that are actually on tracks. But my question is, at what point does a flap system leave one definition and join the other? Is the difference more about the chord lengthening ability of Fowlers vs. slotted, or is it strictly about mechanisms? I could design a flap pivot extension tab with a radius that is so long that it will mimic the chord lengthening abilities of Fowler flaps. So what do you guys think? Is this technically a fowler flap?

Well-Known MemberHBA Supporter

But my question is, at what point does a flap system leave one definition and join the other?

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I don't know if there is an accepted standard definition of a Fowler flap. If there is I'll learn something.
For me, the the separating line between a simple or slotted flap is that the Fowler uses 2 pivot points rather than one. This lets the designer tailor the deployment in almost anyway they want:

More wing area and then increased camber
Increased camber and then more wing area
or a balanced addition of both.

The Fowler method also lets us tuck most of the track structure into the wing reducing drag.

Well-Known Member

I don't know if there is an accepted standard definition of a Fowler flap. If there is I'll learn something.
For me, the the separating line between a simple or slotted flap is that the Fowler uses 2 pivot points rather than one. This lets the designer tailor the deployment in almost anyway they want:

More wing area and then increased camber
Increased camber and then more wing area
or a balanced addition of both.

The Fowler method also lets us tuck most of the track structure into the wing reducing drag.

Click to expand...

I believe the Fowler is defined by its aft movement (increasing wing area) and the slot that funnels air onto its top surface to keep the flow attached. Slotted tracks and pivoted arms under the wing both achieve this. The tracks are a lot more work to manufacture and longer flaps need more than one pushrod to keep things straight, while the pivoted affairs are simpler but are great skull-crackers. Much more drag, too as noted.

Piper used the pivoted flaps on a lot of airplanes, and the Twin Otter has them. Cheap, sturdy, simple and easy to maintain. Cessna's tracked flaps are far more hassle, and I have had many of those apart to repair seized rollers, roller-damaged flap arms,and so on. The flap track slots must be watched for wear. Reassembling them requires many hands, or, with much experience, one guy can do it.

Well-Known Member

I believe definition of the Fowler flap increases wing area over the plane flap, which just pivots on center. A Fowler does have its own leading edge in the airstream, helping keep air attached. A plane flap with a lot of deflection can loose a lot of lift, so it just ends up being a drag maker more than a lift maker. Slotted flaps increase the efficiency of the basic Fowler, but are really modified Fowlers.

Well-Known Member

I can identify hinge flaps. They're pretty clear. They just rotate on a single axis within the body of the flap. My question deals with the vague grey area between slotted and Fowler flaps. At the beginning of the scale is the design where there are 2 or 3 tabs protruding down from the trailing edge to provide a rotation point that creates a turning radius with the axis outside the flap body. At the end are jumbo-jet-style Fowlers that are actually on tracks. But my question is, at what point does a flap system leave one definition and join the other? Is the difference more about the chord lengthening ability of Fowlers vs. slotted, or is it strictly about mechanisms? I could design a flap pivot extension tab with a radius that is so long that it will mimic the chord lengthening abilities of Fowler flaps. So what do you guys think? Is this technically a fowler flap?

Click to expand...

That is a slotted flap. I'd lean towards calling it a Blackburn slotted flap. Blackburn flaps actually used a linkage to simulate a low pivot like the RV uses... The earlier Handley Page flaps had a simple hinge, but not so low. The Blackburn was a development of the Handley page. The Fowler flap is a split flap that moves rearward before lowering. NACA had a play with slotted flaps and came up with their own versions. Fowler flaps slide out the back and greatly increase wing area, but are not the only ones to do that, Zap, Gouge and Fairey-Youngman also slide back and move down at some point. They don't look a whole lot different to Fowlers. Unfortunately, people like calling anything other than a plain flap 'Fowler' now, so the waters are quite muddy.

Have a look at this 1947 paper on flaps. NACA has several papers on flaps too. Read them all!

In the end, it is how well it works, not what it is called. But to design or copy one, it is handy to know what to look for!

Well-Known Member

But my question is, at what point does a flap system leave one definition and join the other?

Click to expand...

A slotted flap uses a single hinge line and usually has a decent sized lip - that allows you to decide just where you want the flap at two particular settings, and the rest of the settings you get what you get as the flap swings about the hinge line. Usually an airfoil shaped flap with maximum angle giving a slot depth and overlap each of about 1% will work well, and the intermediate positions will too. But it is possible to get some intermediate positions that might be less than optimal.

A Fowler flap is on tracks and has a long upper lip. You can usually get more CL this way than with a slotted flap. You can get just what you want in terms of CL and CD for each position and make sure that the lift and drag as you progressively add flaps do not have any cusps or discontinuities that will give strange behavior, either as you add flaps on final or start bringing them up on a go around.

So, have you got a wind tunnel, can afford some more weight to land slower, want to add wing area but only a little camber for the first two notches, then add more camber on the third, and then lots of drag for the last notch? You can seek out the ideal flap position for each one of them with Fowlers, and then design the tracks to give that to you. Or you can go lighter and simpler and accept a stall speed a couple knots faster with slotteds.

Well-Known MemberHBA Supporter

Cessna's tracked flaps are far more hassle, and I have had many of those apart to repair seized rollers, roller-damaged flap arms,and so on. The flap track slots must be watched for wear. Reassembling them requires many hands, or, with much experience, one guy can do it.

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Yeah, if the fowler flap jams, the other flap will still go down with unexpected roll. Be prepared to reverse the switch. Happened to my friend in her 172.

Well-Known Member

What about the manual actuation would prevent asymmetric deployment verses the electrically actuated flaps?

I also prefer the manual actuation.

BJC

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I prefer manual, too, especially in real airplanes like the 180 and 185, but the manual actuation won't prevent a jammed flap. It can prevent the massive damage inflicted by the flap motor if the flap does jam. The jamming is a result of the flap roller edges cutting into the flap support arms; the rollers are just Torrington needle bearings with a thin sleeve pressed over them, and that sleeve tends to work its way off to one end of the roller and starts cutting into the .063" thick flap arm. Once it gets well through the arm the disc punches out and the rib itself is too thin to take the loads, so the bolt cocks and the roller jams in the slot.
29 pages of info here:: https://support.cessna.com/custsupt/contacts/pubs/ourpdf.pdf?as_id=22231

Well-Known MemberHBA Supporter

Big off-topic nitpick here: The graphic shows the RV-10 wing profile correctly, but text underneath it describes it as an "NACA 23013.5". In truth, the RV-10 airfoil is a custom profile designed by my friend Steve Smith, and has almost nothing in common with the NACA profiles used on most of the other RVs.

Well-Known Member

Big off-topic nitpick here: The graphic shows the RV-10 wing profile correctly, but text underneath it describes it as an "NACA 23013.5". In truth, the RV-10 airfoil is a custom profile designed by my friend Steve Smith, and has almost nothing in common with the NACA profiles used on most of the other RVs.

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Well that confirms my suspicion; that is neither a slotted flap nor a Fowler flap, it is a Smith flap.

Well-Known Member

Just to throw another spanner in the works. A split flap that moves aft on extension is a zap flap and predates the fowler flap for which there was a patent. From "The Fowler flap" the definiation is given as varible area, varible camber, a slot between the wing and flap obtained by extending a seperate aerofoil rearward and downward.

Well-Known Member

I don't know if there is an accepted standard definition of a Fowler flap. If there is I'll learn something.
For me, the the separating line between a simple or slotted flap is that the Fowler uses 2 pivot points rather than one. This lets the designer tailor the deployment in almost anyway they want:

More wing area and then increased camber
Increased camber and then more wing area
or a balanced addition of both.

The Fowler method also lets us tuck most of the track structure into the wing reducing drag.

Click to expand...

Flap track design is also modified to control extend/retract forces. Important in manual flap deployment.